The present invention relates to a hydraulic circuit for the control of a drive train of a motor vehicle and relates particularly to a hydraulic circuit for the control of a double clutch transmission which has two transmission groups, each with an isolating clutch and with a plurality of shift clutches for engaging and disengaging of gear steps, the hydraulic circuit having for each transmission group a specific hydraulic branch connected to a pump in each case via a safety valve, and each hydraulic branch having in each case a clutch control valve for controlling the assigned isolating clutch and at least one shift control valve for controlling the assigned shift clutches.
Such a hydraulic circuit is known from DE 101 34 115 A1.
Double clutch transmissions have been known for a long time. Recently, however, there has again been greater interest in double clutch transmissions because the overlapping activation of the two clutches for a gear change free of interruption in traction can nowadays be mastered more easily in regulating terms. This applies particularly to those double clutch transmissions which use wet multiple-disk clutches as isolating clutches.
In general, double clutch transmissions offer a good compromise between a high degree of comfort and high efficiency. As mentioned, owing to the overlapping actuation of the two isolating clutches, it is possible for gears to be changed without any interruption in traction. This is generally not possible, for example, in conventional automated shift transmissions. On the other hand, double clutch transmissions afford a higher efficiency than, for example, classic converter-type automatic transmissions, since double clutch transmissions do not require an energy-consuming hydrodynamic converter.
A hydraulic circuit for the control of such a double clutch transmission is known from DE 101 34 115 A1 initially mentioned. In this case, the hydraulic circuit is divided into branches for the two transmission groups. Each branch has, on the input side, a pilot valve in the form of a nonproportional directional valve. The pilot valve possesses a safety-relevant function, since it makes it possible to “cut out” completely the transmission group which is not active in the respective case.
Each branch has a flow valve with fixed throttling for the control of a cylinder for actuating the assigned isolating clutch. Each transmission group has two shift rods for the actuation of assigned shift clutches. To actuate the shift rods, each hydraulic branch has a proportional pressure valve for each shift rod. The proportional pressure valves and the flow valve are connected to the outlet side of the directional safety valve.
Furthermore, a central directional valve device in the form of a multiplex value is provided between the proportional pressure valves and the four shift rods.
The known hydraulic circuit arrangement has various disadvantages. For example, the known hydraulic circuit has a comparatively large number of valves. The use of flow valves for controlling the isolating clutches causes a valve hysteresis difficult to control in regulating terms and high valve damping. Thus, it is possible, in general, for an oscillating oil column to excite a valve slide of such a valve, and therefore, as a rule, high damping has to be integrated. This leads to losses in dynamics.
Due to the use of one multiplex valve coupling the two hydraulic branches, if this valve fails, the entire double clutch transmission is inoperative.
In general, in hydraulic circuits for drive trains of motor vehicles, it is difficult for safety-relevant devices to be actuated hydraulically. This is because, on account of the high safety requirements, digital regulations, which can be parameterized and have high regulating quality, are usually ruled out.
Safety-relevant devices of a drive train are to be understood as meaning, for example, isolating clutches and shift clutches of an automated shift transmission and of a double clutch transmission, but also variator arrangements for continuously variable transmissions and toroidal transmissions.